The present invention generally relates to a tracking servo system for following up the center of a track with a light spot and an optical disc apparatus in which the tracking servo system is used. More particularly, the invention is concerned with a composite tracking servo system composed compositely of a continuous servo loop in which a differential diffraction method using pregrooves or a so-called push-pull tracking is employed and a sample servo loop in which wobble tracking method using wobbled marks disposed at both sides of a track in a relation wobbled relative to the center of the track, and which system is suitable for application to optical code data memories such as a write-once type optical disc system, an erasable type optical disc system or the like.
As an off-track deviation detecting system adopted widely at present in an optical disc file system, there can be mentioned a so-called push-pull method according to which guide grooves referred to as pregrooves are previously provided on the disc surface, wherein detection of the off-track deviation (hereinafter referred to as the track deviation) is realized by making use of such a phenomenon that upon occurrence of deviation of a light spot focused onto the pregroove from the latter, change will occur in the distribution of the quantity of reflected light due to diffraction of light brought about by the regroove. Although the push-pull system enjoys excellent features preferable for the optical disc file, the system is disadvantageous in respect to high susceptibility to the influence of tilting of the disc, introducing an offset component into the track deviation detecting loop, as the result of which the target or aimed point of the servo system is undesirably shifted, thus making it difficult or even impossible to realize the correct tracking operation. For particulars of the push-pull tracking servo system, reference may be made to, for example, U.S. Pat. No. 4,363,116.